Strain engineered and impurity controlled III-V nitride semiconductor films and optoelectronic devices

ABSTRACT

In the present invention, an interfacial layer is added to a light-emitting diode or laser diode structure to perform the role of strain engineering and impurity gettering. A layer of GaN or Al x In y Gal 1-x-y N (0≦x≦1, 0≦y≦1) doped with Mg, Zn, Cd can be used for this layer. Alternatively, when using Al x In y Ga 1-x-y N (x&gt;0), the layer may be undoped. The interfacial layer is deposited directly on top of the buffer layer prior to the growth of the n-type (GaN:Si) layer and the remainder of the device structure. The thickness of the interfacial layer varies from 0.01-10.0 μm.

FIELD OF THE INVENTION

The present invention relates to the manufacture of optoelectronicdevices, in particular towards the strain engineering and impuritycontrol in the grown layers.

BACKGROUND

Currently, there is no substrate material that can suitably match thelattice constants and thermal expansion coefficients of the compoundsand alloys in the III-V nitride materials system. Thus, the ability togrow high-quality films of the III-V nitrides (AlInGaN) by standardepitaxial techniques (e.g., organometallic vapor phase epitaxy (OVPE),molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE)) onmismatched substrates, like sapphire and silicon carbide, is a keycomponent to producing high-quality layers and achieving optimal deviceperformance. Growth of AlInGaN layers at typical growth temperatures(>1000° C.) results in films consisting of a mosaic assemblage ofhexagonal nuclei. These layers exhibit a very rough morphology, veryhigh background donor concentrations and are prone to cracking.

Using nucleation or buffer layers, deposited at low temperature(400-900° C.) on sapphire and at higher temperatures on silicon carbide,prior to high temperature growth, allows the crystal grower todramatically improve the quality of epitaxial nitride films. Commonly,these buffer layers consist of AlN, GaN or some composition intermediateto these two binaries. The insertion of this low temperature bufferlayer provides the means by which drastic differences in: 1) latticeparameter, 2) thermal expansion, 3) surface energy and 4)crystallography between the substrate, e.g. sapphire, and the nitrideepilayer are overcome.

Nitride-based light-emitting diodes (LEDs) typically include asubstrate, a nucleation or buffer layer, an n-type conducting layer, anactive layer, a p-type conducting layer, and metal contacts to the n-and p-type layers. A schematic of a generic LED is shown in FIG. 1.Nitride LEDs typically have the structure shown in FIG. 2. Thenucleation layer is commonly AlN, GaN or AlGaN.

An added complication when dealing with nitride epitaxy is the problemof cracking. Cracking arises when the epitaxial films are pulled intension either due to: 1) lattice mismatch between substrate and film,2) thermal expansion coefficient mismatch between substrate and film, 3)high doping levels and 4) lattice mismatch due to intentionalcompositional modulations during the growth of a nitride device. Typicalnitride-based devices exhibit heavily doped layers, where the dopantconcentrations often exceed 10¹⁸-10¹⁹ cm⁻³, and several compositionalheterointerfaces. Data for the lattice parameter and thermal expansioncoefficient for the nitrides and the common substrates (SiC andsapphire) are shown below in Table I.

TABLE I Properties of nitrides and selected substrates. Material GaN AlNInN sapphire 6H-SiC Lattice Constant (Å) a 3.129 3.112 3.548 4.758 3.08c 5.185 4.982 5.76 12.991 15.12 Thermal Expansion Coefficient (/K¹) a5.59 × 10⁻⁶  4.2 × 10⁻⁶ 4 × 10⁻⁶ 7.5 × 10⁻⁶  4.2 × 10⁻⁶ c 3.17 × 10⁻⁵5.59 × 10⁻⁶ 3 × 10⁻⁶ 8.5 × 10⁻⁶ 4.68 × 10⁻⁶

While the problems associated with lattice- and thermal-mismatch can beadequately addressed using existing nucleation layer technologies and bycontrolling the heating and cooling conditions associated with growth,cracking due to doping and intentional compositional fluctuations cannotbe solved by such methods.

Cracking presents a considerable problem when GaN layers are dopedn-type with Si (which has an ionic radius more than 30% smaller than Ga,the atom for which it substitutes) and when layers of differingcompositions are deposited on one another. The second case is especiallytroublesome when the layer grown on top has a smaller a-axis latticeparameter than the layer on which it is grown, e.g. AlN or AlGaNdeposited on GaN, due to the very rigid elastic constants exhibited bythe III-V nitrides. Additionally, heterostructures consisting of nitridelayers generally exhibit registry along the a-axis, which is parallel tothe substrate film interface, and are distorted only along the c-axis,which is perpendicular to the substrate film interface. Thus, when alayer has a smaller relaxed a-axis parameter than the layer on which itis grown, tensile stress is induced in that layer in order to keep theinterface in registry.

Another problem that the crystal grower typically encounters is that ofunwanted impurities in otherwise pure crystals. Among the commonimpurities that can occur during the process of crystal growth,regardless of the method employed, oxygen is generally considered to bethe most troublesome. Oxygen can severely limit the grower's ability tocontrol conductivity, strain and optical luminescence. Sources of oxygencan include, but are not limited to, reactant sources, reactor walls andhardware, graphite susceptors or boats and even the substrate wafersthemselves.

SUMMARY

In the present invention, an interfacial layer is added to alight-emitting diode or laser diode structure to perform the role ofstrain engineering and impurity gettering. A layer ofAl_(x)In_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1) doped with Mg, Zn, Cd can be usedfor this layer. Alternatively, when using Al_(x)In_(y)Ga_(1-x-y)N (x>0),the layer may be undoped. The interfacial layer is deposited directly ontop of the buffer layer prior to the growth of the n-type (GaN:Si) layerand the remainder of the device structure. The thickness of theinterfacial layer varies from 0.01-10.0 μm.

The interfacial layer increases device reliability and reproducibilitybecause the problems associated with cracking, layer coalescence, andimpurity trapping are relegated to a region of the device that is notactive during device operation. To illustrate, the interfacial layer“getters” or traps the residual impurities (such as O) in the initiallayer of the structure. In addition, this process also cleanses thechamber and the reactor components making them free of undesiredimpurities which would be present later when the more critical layers,e.g. the active layer or the p-type layers, in the structure are grown.The preferred embodiments for this layer include GaN:Mg and AlGaN forthe composition of the interfacial layer because both Mg and Al have ahigh affinity for oxygen. Additionally, the use of this interfaciallayer reduces the strain and lessens the driving force for cracking bychanging the nature of the strain state of the nitride epilayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a generic light-emitting diode of the prior art.

FIG. 2 illustrates a typical nitride-based LED of the prior art.

FIG. 3 illustrates a light-emitting diode of the present invention.

FIG. 4 shows a SIMS profile of a GaN:Mg layer where the presence of O atthe interface can be clearly seen.

FIG. 5 illustrates the depth profile for Mg of a prior art LED.

FIG. 6 illustrates the depth profile for Mg using the method of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 3 illustrates an embodiment of the present invention 10. Aninterfacial layer 16 is added to a light-emitting diode or laser diodestructure to performs the role of strain engineering and impuritygettering. A layer of Al_(x)In_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1) doped withMg, Zn, Cd can be used for the interfacial layer. Alternatively, whenusing Al_(x)In_(y)Ga_(1-x-y)N with x>0, the interfacial layer may beundoped. The interfacial layer can also include alloys of AlInGaN,AlInGaP, and AlInGaAs, and alloys of GaN, GaP, and GaAs. The interfaciallayer 16 is deposited directly on top of the buffer layer 14 prior tothe growth of the n-type (GaN:Si) layer 18, active region 10, and thep-type layer 22. The thickness of the interfacial layer varies from0.01-10.0 μm, having a preferred thickness range of 0.25-1.0 μm. Bufferlayer 14 is formed over a substrate 12. Substrate 12 may be transparent.Metal contact layers 24A, 24B are deposited to the p-type and n-typelayers 22, 18, respectively.

The interfacial layer increases device reliability and reproducibilityby “gettering” or trapping the residual impurities, e.g., oxygen, in theinitial layer of the structure. The process also cleanses the chamberand the reactor components making them free of further impurities whichwould be present later when the more critical layers, e.g., the activelayer or the p-type layers, in the structure are grown. The preferredembodiment uses GaN:Mg and/or AlGaN for the composition of theinterfacial layer because both Mg and Al have a high affinity foroxygen. Generally, the sources that are affected adversely by thepresence of oxygen-containing impurities, e.g. those containing Mg, Zn,and Al, are easier to use and less prone to pre-reactions and,ultimately, gas-phase depletion after the interfacial layer is grown.

FIG. 4 shows a secondary ion mass spectrometry (SIMS) profile of aGaN:Mg layer where the presence of oxygen at the interface can beclearly seen. After the first 0.25-0.35 μm, the concentration of oxygenis reduced to the SIMS background level, indicating that the oxygen hasbeen trapped in this non-critical portion of the structure. FIG. 5 showsthe Mg profile in a GaN-based LED. The Mg-doped region on the right sideof the figure is the interfacial layer described in this invention. TheIn profile is provided as a marker, indicating the location of theactive region.

When the GaN:Si layer is directly deposited on the buffer layer (as istypical for GaN-based optoelectronic devices), cracking tends to be aproblem. Si has a smaller atomic radius than that of Ga (0.41 vs. 0.62Å), which Si displaces from the lattice. Films doped with Si are grownin a state of tension, an unfavorable state for brittle materials, e.g.GaN. The size of the Mg and Zn ionic radii are comparable to that of theatom for which they substitute when doping (Ga=0.62 Å; Mg=0.66 Å;Zn=0.74 Å). Additionally, the ionic radius of Cd is 0.94 Å.Incorporating these atoms into the GaN layer shifts the strain stateassociated with dopant impurities to that of compression, a veryfavorable state for GaN. Similarly, for GaN grown on AlGaN, where theAlGaN has a smaller lattice constant than GaN, the GaN layer will be ina state of compression and result in a significant reduction incracking.

In the prior art, one major problem seen in all manufacturing processesusing Mg as a dopant source is the issue of turn-on time. Due to itsreactive nature and strong attraction to moisture and oxygen as well asthe reactor plumbing and walls, Mg is often a difficult impurity tocontrol during crystal growth. Often, the chemical profile for Mg takesan extended time and substantial film thickness before reaching anequilibrium concentration. Since the carrier mobility and lifetime forholes in GaN:Mg are generally low, the placement of Mg, and hence thelocation of the p-n junction is critical for efficient LED operation.Since the thickness of the interfacial layer is generally larger thanthe thickness required to achieve the equilibrium concentration ofmagnesium, the present invention can be used to greatly reduce the timeneeded to reach equilibrium concentration. The result is a much sharperMg profile, producing a sharp junction between the n- and p-type layersin the structure and improving the device efficiency. A comparisonbetween profiles using the prior art and the method described here arepresented in FIG. 5. Both y-axes are normalized to reflect an absoluteMg concentration of 5×10¹⁸-5×10²¹ cm⁻³.

We claim:
 1. A light-emitting diode comprising: a substrate; a bufferlayer formed over the substrate; an interfacial layer, formed over thebuffer layer, including a dopant that has an affinity for oxygen-bearingcompounds; an n-type layer, formed directly on the interfacial layer; anactive region, formed over the n-type layer; a p-type layer, formed overthe active region; and two electrical contacts, one of the two beingconnected to the n-type layer and the other of the two being connectedto the p-type layer.
 2. A light-emitting diode, as defined in claim 1,wherein the interfacial layer is selected from the group consisting ofalloys of AlInGaN, AlInGaP, and AlInGaAs.
 3. A light-emitting diode, asdefined in claim 1, wherein the interfacial layer is selected from thegroup consisting of alloys of GaN, GaP, and GaAs.
 4. A light-emittingdiode, as defined in claim 1, wherein the dopant has an ionic radius onthe order of a radius of an atom being substituted.
 5. A light-emittingdiode, as defined in claim 1, wherein the dopant is selected from thegroup consisting of Mg, Zn, and Cd.
 6. A light-emitting diodecomprising: a transparent substrate; a buffer layer formed over thetransparent substrate; an interfacial layer, formed over the bufferlayer, including a dopant that has an affinity for oxggen-bearingcompounds; a first layer, having a first type, formed over theinterfacial layer; an active region, formed over the first layer; asecond layer, having a second type, formed over the active region; and afirst and second electrical contact, the first contact being connectedto the first layer and the second contact being connected to the secondlayer.
 7. A light-emitting diode, as defined in claim 6, the interfaciallayer further comprising a dopant having an ionic radius similar to aradius of an atom that it displaces.